Conventional Handover/Measurement Report
Conventionally, a mobile terminal is connected to a single cell. When it moves from the serving area of one cell to the serving area of another cell, typically a handover is initiated. The full picture of all cells in the surrounding of the terminal is only available at this terminal itself. However, a general paradigm of a well-organized network is that the network makes the mobility decisions, and not the terminal.
One solution would be that the terminal permanently sends measurement reports to the network. However, this is too much overhead in terms of required signalling. Instead, the 3rd Generation Partnership Project (3GPP) specification allows configuring triggers for the terminal. If such a network-configured trigger expires in the terminal, the terminal will send a measurement report. It is recommendable that the network configures the triggers such that a handover is initiated when such a measurement report is received. This minimizes the number of measurement reports.
For intra-frequency handovers in LTE, the most prominent trigger for a measurement report is the A3 trigger which is defined as follows (simplified):Mn+Ocn>Ms+Off  Eq. (1)
It expires if the measurement Mn of a neighbour n is offset Off better than the measurement Ms of a serving cell. The measurements could be given as signal strength or as a signal quality.
The offset Off introduces a kind of hysteresis to the handover decision to avoid ping-pongs.
Ocn is another offset (also called “cell individual offset”) which, in contrast to Off, is neighbour-specific. It can be used to fine-adjust the handovers individually towards different neighbours due to mobility robustness reasons (e.g. make the neighbour more attractive if it is entered through a high-speed street), or due to load balancing reasons (e.g. make the neighbour more attractive if it is low loaded).
Multi-Connectivity
With multi-connectivity there is not only a single serving cell, but there are multiple serving cells. In this application, the set of serving cells is sometimes called “active set”.
The active set is UE-specific and is configured by the network using the measurements reports sent by the UE. As the UE moves, new detected cells will be added to the active set and others will be removed from the active set as their signals become weaker.
FIG. 1 shows an example of an active set consisting of three cells 1001, 1002, 1003 serving a terminal 1010 (serving is indicated by solid arrows). The terminal 1010 is moving in the direction of dashed arrow 1011 and approaches cell 1004, which is not in the active set. The network may decide to add cell 1004 to the active set and/or to remove cell 1001, and reconfigures the terminal with a new active set.
In one of the multi-connectivity schemes foreseen for 5G systems using OFDMA as physical layer technology, all the cells of the active set will transmit synchronously the same signal on data and control channels and on the same time and frequency resource. This transmission scheme provides a combining gain to Signal-to-Interference and Noise Ratio (SINR), and consequently, improves the mobility of the UE by reducing Radio Link Failures (RLFs) and enhances cell-edge throughput [1]. This solution is different from LTE “Cooperative Multi-Point” (CoMP) transmission where the control plane is handled only by one cell of the active set called Primary Cell (PCell), i.e., no gain is provided on control channel, and the connection of the UE depends purely on the link of PCell.
This low layer multi-connectivity scheme is denoted in the sequel by “Single-Frequency Network” (SFN) transmission in analogy to enhanced Multimedia Broadcast Multicast System (eMBMS) where all the cells belonging to a specific Multimedia Broadcast SFN (MBSFN) area transmit the same signal in MBMS subframes.
Some prior art is described in the following:
1. Active Set Configuration for 3G Soft Handover
3GPP has already defined triggers for configuring and maintaining an active set for 3G Soft Handover. For instance, 3GPP TS 25.331 has defined the “1A” event which is defined as (simplified)
                              Mn          +          Ocn                >                                            max                              s                ∈                A                                      ⁢                          (              Ms              )                                +                      Off            add                                              Eq        .                                  ⁢                  (          2          )                    
where the symbol A is used for the active set, i.e. A is a set consisting of a number of serving cells s1, s2, . . . .
Such a condition expires if the measurement Mn of a “new” cell n is offset better then the measurement of the best cell in the current active set. The expiry would trigger a measurement report to the base station, and the base station would add the new cell n to the active set.
Here, in contrast to a typical hysteresis value used in single-connectivity, Offadd is likely to be a negative value, i.e. you would add a cell to the active set, even if it is still weaker then the best cell.
Equivalent to the definition of 1A trigger event, there exists a second trigger event 1B to remove a cell from the active set. The network removes a cell s0 from the active set A when a measurement report is received which has been triggered by the following condition:
                                          Ms            ⁢                                                  ⁢            0                    +                      Ocs            ⁢                                                  ⁢            0                          <                                            max                              s                ∈                A                                      ⁢                          (              Ms              )                                +                      Off            remove                                              Eq        .                                  ⁢                  (          3          )                    
i.e. a cell would be removed if it falls significantly below the best cell (for a certain time to trigger).
In order to avoid the alternating addition and removal of the same cell (similar to a ping-pong), there should be a difference between Offadd and Offremove.
Example: Ocn=0; Ocs0=0; (for simplicity); Offadd=−6 dB; Offremove=−8 dB:                A cell would be added using event 1A when it comes closer than 6 dB to the best cell:        
  Mn  >                    max                  s          ∈          A                    ⁢              (        Ms        )              -          6      ⁢                          ⁢      dB                      A cell would be removed using event 1B when it falls more than 6 dB below the best cell:        
  Mn  <                    max                  s          ∈          A                    ⁢              (        Ms        )              -          8      ⁢                          ⁢      dB      This condition is to be fulfilled for a certain time period (“time to trigger”), whereas in every time instance the condition is evaluated using the best of all cells. There is also an event 1C defined in 3GPP TS 25.331 for replacing the weakest cell in the active set with a better one not in the active set.
These triggers defined for 3G Soft Handover ensure that the relevant cells, in terms of signal strength/quality, are in the active set of the UE. The received signals of these cells are combined at the UE using the Rake receiver which compensates for the difference in the arrival times of the signals. Thus, in 3G systems the receiver does not have the same constraint imposed by the CP in OFDM-based network where combining gain is achieved only if the signals of different cells are received within the CP duration.
2. Extension of CP and Re-Use of Triggers Defined for 3G Soft Handover
The frame structure foreseen for 5G cmWave technology for ultra-dense small cell deployments is shown in FIG. 4. The current frame numerology for 5G cmWave technology assumes one subframe of 250 μs consisting of 14 OFDM symbols with total symbol duration Ts=Tu+TCP=16.67+1=17.67 μs and 3 Guard Period (GP), each of 0.89 μs. The first two symbols are used for downlink (DL) and uplink (UL) control channels whereas the rest 12 OFDM symbols are used either for DL or UL data transmission.
The sub-carrier spacing is 1/Tu=60 kHz and the total overhead of CP and GP is (14*1 μs+3*0.89 μs)/250 μs=6.67%.
In principle, the CP could be extended to ensure that the signals of the cells in the active set, selected using the defined triggers for 3G soft handover, do not cause ISI. However, the value of the new CP will have some dependency on the value of the negative offset Offadd of Eq. (2): The lower the add offset Offadd, the higher the probability is of adding cells whose signals are falling outside the FFT window and causing ISI. As such, to avoid multiple CP designs the CP should be extended to capture the worst-case scenario corresponding to the lowest value configurable for Offadd if it is known beforehand. This requirement is challenging considering the drawbacks of extending the CP which are described in the following.
If the overhead of CP and GP is to be kept constant, extension of CP would be at the expense of a more vulnerable system with respect to Inter-Carrier Interference (ICI) and reduction in supported user mobility speed. For instance, consider the case of doubling the CP from 1 μs to 2 μs. To keep the same overhead ratio of CP and GP, 7 OFDM symbols with symbol duration of Ts=Tu+TCP=33.33 μs+2 μs=35.33 us would fit in one TTI of 250 μs. As such, the sub-carrier spacing is reduced from 60 kHz to 1/33.33 μs=30 kHz leading to halving the maximum tolerable Doppler spread and in turn the supported user velocity.
On the other hand, if the subcarrier spacing is kept fixed to 60 kHz and CP is doubled from 1 μs to 2 μs, the new total symbol duration Ts would be equal to Tu+TCP=16.67 μs+2 μs=18.67 μs leading to an increase in the overhead of CP with respect to Ts from 1 μs/17.67 μs=5.66% to 2 μs/18.67 μs=10.71%.
Extension of CP has then drawbacks on either the supported user velocity or on the overhead and capacity of the system.
Moreover, there is another problem that can still occur and cannot be solved even if the CP had been extended: A UE configured with an active set of cells has to constantly update the position of the FFT window to account for the changes in the delays of the received signals caused by UE movement or by variations in the environment. As such, it could happen that at some time instant some of the cells in the active set start to generate significant amount of ISI due to a shift in the position of the FFT window. Using prior-art scheme defined for 3G soft handover, these cells cannot be removed from the active set as long as the measurement event 1B (or similar event) for removing a cell did not expire.